Method of cutting semiconductor wafer and protective sheet used in the cutting method

ABSTRACT

In a method of cutting a semiconductor wafer in which the semiconductor wafer  6  is cut by plasma etching, a protective sheet  30  on which a metallic layer  30   b , a plasma etching rate of which is low, is formed on one face of an insulating sheet  30   a  is stuck on to a circuit forming face  6   a  by an adhesive layer  30   c , and plasma is exposed onto an opposite side to the circuit forming face  6   a  from a mask side which is formed by covering regions except for cutting lines  31   b  with a resist film  31   a  so as to conduct plasma etching on portions of the cutting lines. Due to the above structure, it is possible to use the metallic layer as an etching stop layer for suppressing the progress of etching. Therefore, fluctuation of the progress of etching can be avoided and heat damage caused on the protective sheet can be prevented.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a method of cutting asemiconductor wafer by means of plasma etching. The present inventionalso relates to a protective sheet used in the method of cutting thesemiconductor wafer.

[0002] A semiconductor device mounted on a board of electronic equipmentis conventionally manufactured in such a manner that pins of a leadframe and metallic bumps are connected to semiconductor elements, onwhich a circuit pattern is formed in the state of a wafer, and thesemiconductor elements are subjected to a packaging process so that theycan be sealed with resin. Since the size of electronic equipment hasbeen recently reduced, the size of the semiconductor device has beenalso decreased. Especially, investigations are actively made into thereduction of the thickness of a semiconductor element.

[0003] The mechanical strength of the semiconductor element, thethickness of which is reduced, is so low that the semiconductor elementis liable to break in the process of cutting conducted in the dicingstep in which the semiconductor element in the state of a wafer is cutinto individual pieces, and the yield of machining is inevitablylowered. Concerning the method of cutting the semiconductor element, thethickness of which is reduced, instead of the mechanical cutting method,a plasma dicing method is proposed in which the semiconductor wafer iscut when cutting grooves are formed by the etching action of plasma.Concerning this method, for example, refer to Japanese PublicationJP-A-2002-93752.

[0004] However, in the process of plasma dicing of the prior artdescribed above, due to the want of uniformity of etching actionconducted by plasma, the following problems are caused and have not beensolved yet. In the process of plasma etching, masking is conducted on asemiconductor wafer in which regions except for the cutting lines arecovered with a resist layer. After the completion of masking, thesemiconductor wafer is accommodated in a processing chamber in a plasmaprocessing device, and only regions of the cutting lines are exposed toplasma in the processing chamber so as to remove these portions by meansof etching.

[0005] In this connection, an etching rate showing the degree of etchingconducted by plasma is not necessarily uniform. Therefore, the etchingrate distribution fluctuates in the processing chamber. Accordingly, inthe process of plasma dicing conducted in the processing chamber,silicon in the portions of the cutting lines, which are located in arange of a high etching rate, is more quickly removed than silicon inthe other portions. Therefore, cutting is more quickly completed inthese portions.

[0006] The cutting lines in these portions of the high etching rate, aresuccessively exposed to plasma until silicon in the portions of thecutting lines located in regions of the low etching rate is removed.Accordingly, when silicon is completely removed from the regions of thehigh etching rate, the protective sheet on the lower face side of thesemiconductor wafer is directly exposed to plasma.

[0007] When the plasma processing continues in the above state, heatgenerated by plasma directly acts on the protective sheet. As a result,there is a possibility that the protective sheet is superheated, burnedand deformed. According to the conventional plasma dicing method, it isimpossible to effectively prevent the protective sheet from beingdamaged by heat caused by the want of uniformity of the etching actionof plasma.

SUMMARY OF THE INVENTION

[0008] Therefore, it is an object of the present invention to provide amethod of cutting a semiconductor wafer capable of preventing aprotective sheet from being damaged by heat when the semiconductor waferis cut by plasma etching. It is also an object of the present inventionto provide the protective sheet used in the method of cutting thesemiconductor wafer.

[0009] According to the present invention, a method of cutting asemiconductor wafer in which the semiconductor wafer, on the first faceof which semiconductor elements are formed, is cut by plasma etchingfrom the second face opposite to the first face, comprises: a sheetattaching step of attaching a protective sheet onto the first face; amask formation step of forming a mask to determine cutting lines on thesecond face for cutting the semiconductor wafer; and a plasma etchingstep of conducting plasma-etching on portions of the cutting lines byexposing the semiconductor wafer to the plasma from the mask side,wherein the protective sheet is composed of an insulating sheet, whichbecomes the base material, and a metallic layer, a plasma etching rateof which is lower than that of the semiconductor wafer, provided on oneface of the insulating sheet, and the metallic layer side is attachedonto the first face via an adhesive layer in the sheet attaching step.

[0010] Preferably, plasma generating gas used in the plasma dicing stepmay contain at least gas of fluorine, and the metallic layer may containeither aluminum or copper.

[0011] According to another aspect of the present invention, it isprovided a protective sheet used in a method of cutting a semiconductorwafer, the protective sheet being attached onto a first face of thesemiconductor wafer when the semiconductor wafer, on the first face ofwhich semiconductor elements are formed, is cut by plasma etching fromthe second face opposite to the first face, the protective sheetcomprising: an insulating sheet that becomes the base material; and ametallic layer provided on one face of the insulating sheet and made ofmetal, a plasma etching rate of which is lower than that of thesemiconductor wafer.

[0012] Preferably, plasma generating gas used in the plasma etching maycontain at least gas of fluorine, and the metallic layer may containeither aluminum or copper.

[0013] Further, an adhesive layer may be provided on a face of themetallic layer.

[0014] According to the present invention, in a method of cutting asemiconductor wafer by plasma etching, a protective sheet is used, onwhich a metallic layer, a plasma etching rate of which is lower thanthat the semiconductor wafer, is formed on one face of an insulatingsheet which is the base material, and this metallic layer is utilized asan etching stop layer for suppressing the progress of plasma etching.Due to the foregoing, it is possible to realize operation of effectiveplasma etching in which fluctuation of the progress of plasma etching isavoided, so that the occurrence of heat damage given to a protectivesheet in the process of cutting can be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a cross sectional side view of a plasma processingdevice of an embodiment of the present invention;

[0016]FIG. 2 is a partially cross sectional view of a lower electrode ofthe plasma processing device of the embodiment of the present invention;

[0017] FIGS. 3(a) and 3(b) are cross sectional views of the plasmaprocessing device of the embodiment of the present invention;

[0018] FIGS. 4(a) to 4(e) are schematic illustration for explaining aprocess of method of manufacturing the semiconductor device of theembodiment of the present invention;

[0019]FIG. 5 is a flow chart of the plasma processing method of theembodiment of the present invention; and

[0020] FIGS. 6(a) to 6(c) is a schematic illustration for explaining aprocess of method of manufacturing the semiconductor device of theembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Next, referring to the drawings, an embodiment of the presentinvention will be explained below.

[0022] First, referring to FIGS. 1 to 3(b), the plasma processing devicewill be explained below. This plasma processing device is used in themanufacturing process of a semiconductor device obtained in such amanner that a semiconductor wafer, on the circuit forming face (thefirst face) of which a plurality of semiconductor elements are formed,is divided into individual piece of the semiconductor elements so as toobtain a semiconductor device, the thickness of which is not more than100 μm.

[0023] In the manufacturing process of this semiconductor device, firstof all, a protective sheet made of material, a plasma etching rate ofwhich is lower compared with silicon that is a primary material of thesemiconductor, is attached to the circuit forming face of thesemiconductor wafer. On the reverse face opposite to the circuit formingface of the semiconductor wafer, a mask to determine cutting lines usedfor dividing the semiconductor wafer into individual pieces of thesemiconductor elements is formed. The step of plasma dicing is conductedon the above semiconductor wafer by the present plasma processingdevice.

[0024] In FIG. 1, the processing chamber 2 for conducting plasmaprocessing on the semiconductor wafer is provided inside the vacuumchamber 1. This processing chamber 2 forms a tightly sealed space forgenerating plasma in the state of reduced pressure. In a lower portioninside the processing chamber 2, the lower electrode 3 is arranged. Inan upper portion of the lower electrode 3, the upper electrode 4 isarranged being opposed to the lower electrode 3. The lower electrode 3and the upper electrode 4 are respectively formed into a cylindricalshape and arranged in the processing chamber 2 concentrically with eachother.

[0025] The lower electrode 3 is made of conductive material such asaluminum. The profile of the lower electrode 3 is formed in such amanner that the supporting portion 3 b is extended downward from thedisk-shaped electrode portion 3 a. When the supporting portion 3 b isheld by the vacuum chamber 1 via the insulating material 5C, the lowerelectrode 3 is attached being electrically insulated. The upperelectrode 4 is made of conductive material such as aluminum in the samemanner as that of the lower electrode 3. The support portion 4 b isextended upward from the disk-shaped electrode portion 4 a.

[0026] The support portion 4 b is electrically continued to the vacuumchamber 1 and can be elevated by the electrode elevating mechanism 24not shown in the drawing. Under the condition that the upper electrode 4is lowered, an electric discharge space for generating a plasma electricdischarge used for plasma processing is formed between the upperelectrode 4 and the lower electrode 3. The electrode elevating mechanism24 functions as an electrode distance changing means. When the upperelectrode 4 is elevated by the electrode elevating mechanism 24, theelectrode distance between the lower electrode 3 and the upper electrode4 can be changed.

[0027] Next, explanations will be made into the structure of the lowerelectrode 3 and the semiconductor wafer to be processed. An upper faceof the electrode portion 3 a of the lower electrode 3 is a plane-shapedholding face on which the semiconductor wafer is put. The insulatingcoating layer 3 f is provided in an outer peripheral portion of theholding face. The insulating coating layer 3 f is made of ceramics suchas aluminum. Due to the above structure, the outer peripheral portion ofthe lower electrode 3 is insulated from plasma generated in the electricdischarge space 2 b, so that the occurrence of an abnormal electricdischarge can be prevented.

[0028]FIG. 2 is a view showing a state in which the semiconductor wafer6 before the start of plasma dicing is put on the lower electrode 3. Thesemiconductor wafer 6 is a semiconductor substrate, the primarycomponent of which is silicon. The protective sheet 30 is attached tothe circuit forming face (the first face) of the surface (the lower facein FIG. 2) of the semiconductor wafer 6. Under the condition that thesemiconductor wafer 6 is put on the lower electrode 3, the protectivesheet 30 is tightly contacted with the holding face 3 g of the electrodeportion 3 a.

[0029] The protective sheet is composed in such a manner that a metalliclayer, a plasma etching rate of which is than that of the semiconductorwafer 6, for example, the metallic layer 30 b containing either aluminumor copper is formed on one face of the insulating sheet 30 a of the basematerial and the adhesive layer 30 c is provided on a surface of themetallic layer 30 b as shown in FIG. 6. In the case of attaching theprotective sheet 30 onto the semiconductor wafer 6, the metallic layer30 b side is attached onto the circuit formation face via the adhesivelayer 30 c.

[0030] Due to the foregoing, even if the etching rate of thesemiconductor wafer partially fluctuates since the etching ratedistribution by plasma is not uniform in the process of plasma dicing,the metallic layer 30 b functions as an etching stop layer.

[0031] The insulating sheet 30 a is a resin sheet made of insulatingresin such as polyolefine, polyimide or polyethylene terephthalate andformed into a film of 100 μm thickness. Under the condition that theprotective sheet 30 is attached to the semiconductor wafer 6, theinsulating sheet 30 a functions as a dielectric body when thesemiconductor wafer 6 is electrostatically attracted to the holding face3 g of the electrode portion 3 a.

[0032] On the reverse face (the second face) on the opposite side (theupper side in FIG. 2) to the circuit forming face, a mask fordetermining the cutting lines in the process of plasma dicing describedlater is formed. This mask is formed when patterning is conducted with aresist film after the reverse face is machined as described later. Dueto the foregoing, a region except for the portion of the cutting line 31b, which is an object of plasma etching, is covered with the resin film31 a.

[0033] As shown in FIG. 2, a plurality of attracting holes 3 e, whichare open to the holding face 3 g, are provided in the lower electrode 3.These attracting holes 3 e are communicated with the suction holes 3 cprovided in the lower electrode 3. As shown in FIG. 1, the suction holes3 c are connected to the vacuum suction pump 12 via the gas linechangeover valve 11. The gas line changeover valve 11 is connected tothe N₂ gas supply section 13 for supplying nitrogen gas. When gas linechangeover valve 11 is changed over, the suction holes 3 c can beconnected being selectively changed over between the vacuum suction pump12 and the N₂ gas supply section 13.

[0034] When the vacuum pump 12 is driven under the condition that thesuction holes 3 c are communicated with the vacuum suction pump 12,vacuum suction is conducted from the attracting holes 3 e and thesemiconductor wafer 6, which is put on the lower electrode 3, isattracted by vacuum and held. Accordingly, the attracting holes 3 e, thesuction holes 3 c and the vacuum suction pump 12 compose the attractingand holding means for holding the semiconductor wafer 6 under thecondition that the protective sheet 30 is tightly contacted with theholding face 3 g of the electrode portion 3 a when vacuum suction isconducted from the attracting holes 3 e which are open to the holdingface 3 g of the lower electrode 3.

[0035] When the suction holes 3 c are connected to the N₂ gas supplysection 13, N₂ gas can be blown out from the attracting holes 3 e to thelower face of the protective sheet 30. As described later, this N₂ gas,which is blown out from the attracting holes 3 e to the lower face ofthe protective sheet 30, is blown out for the object of forciblydetaching the protective sheet 30 from the holding face 3 g.

[0036] A coolant flow passage 3 d in which coolant used for coolingflows is provided in the lower electrode 3. The coolant flow passage 3 dis connected to the cooling mechanism 10. When the cooling mechanism 10is driven, coolant such as cooling water circulates in the coolant flowpassage 3 d. Therefore, the lower electrode 3 and the protective sheet30 on the lower electrode 3, the temperatures of which are raised byheat generated in the process of plasma processing, are cooled by thecirculating coolant. The coolant flow passage 3 d and the coolingmechanism 10 compose the cooling means for cooling the lower electrode3.

[0037] The exhaust port 1 a, which is communicated with the processingchamber 2, is connected to the vacuum pump 8 via the exhaust changeovervalve 7. When the exhaust changeover valve 7 is changed over and thevacuum pump 8 is driven, the inside of the processing chamber 2 of thevacuum chamber 1 is exhausted by vacuum, so that the pressure in theprocessing chamber 2 can be reduced. The processing chamber 2 isprovided with a pressure sensor 28 which is omitted in the drawing. Whenthe vacuum pump 8 is controlled according to the result of measuring thepressure by this pressure sensor 28, the pressure in the processingchamber 2 can be reduced to a desired value. The vacuum pump 8 composesa pressure reducing means for reducing the pressure in the processingchamber 2 to a desired value. When the exhaust changeover valve 7 ischanged over to the atmospheric air side, the atmosphere is introducedinto the processing chamber 2, and the pressure in the processingchamber 2 can be returned to the atmospheric pressure.

[0038] Next, the upper electrodes 4 will be explained in detail. Theupper electrodes 4 includes: a central electrode 4 a; and an extendingportion 4 f made of insulating material which surrounds the electrodeportion 4 a and extends to the outer circumferential portion of thecentral electrode 4 a. The profile of the extending portion 4 f islarger than that of the lower electrode 3 and arranged being extendedoutside the lower electrode 3. A gas blowing portion 4 e is provided atthe central portion on the lower face of the upper electrode 4.

[0039] The gas blowing portion 4 e supplies gas for generating plasmawhich is used for generating plasma electric discharge in the electricdischarge space formed between the upper electrode 4 and the lowerelectrode 3. The gas blowing portion 4 e is a circular plate member madeof porous material having a large number of minute holes in it. Gas forgenerating plasma is uniformly blown out from the gas staying space 4 ginto the electric discharge space via the minute holes so that gas canbe uniformly supplied.

[0040] A gas supply hole 4 c communicating with the gas staying space 4g is provided in the support portion 4 b. The gas supply hole 4 c isconnected to the first plasma generating gas supply section 21 via thegas flow rate adjusting section 19 and the gas opening and closing valve20. The plasma generating gas supply section 21 supplies mixed gascontaining fluorine gas such as mixed gas in which sulfur hexafluoride(SF₆) or carbon tetrafluoride (CF₄) is mixed with helium gas.

[0041] When the gas opening and closing valve 20 is opened, it ispossible to supply the plasma generating gas from the plasma generatinggas supply section 21 into the electric discharge space 2 b via the gasblowing section 4 e.

[0042] In the above plasma generating gas supply system, when the gasflow rate adjusting section 19 is controlled according to a command sentfrom the control section not shown in the drawing, a flow rate of gassupplied into the electric discharge space 2 b can be arbitrarilyadjusted. Due to the foregoing, pressure in the processing chamber 2,into which plasma generating gas is supplied, is controlled according tothe plasma processing condition, which has been previously set, andaccording to the pressure in the processing chamber 2 detected by thepressure sensor. Accordingly, the gas flow rate adjusting section 19composes the pressure control means for controlling the pressure in theprocessing chamber 2.

[0043] The lower electrode 3 is electrically connected to the highfrequency electric power supply section 17 via the matching circuit 16.When the high frequency electric power supply section 17 is driven, ahigh frequency voltage is impressed between the upper electrode 4, whichis electrically continued to the vacuum chamber 1 grounded to thegrounding section 9, and the lower electrode 3. Due to the foregoing,plasma electric discharge is generated in the electric discharge space 2b between the upper electrode 4 and the lower electrode 3. Accordingly,the plasma generating gas supplied to the processing chamber 2 istransferred into a state of plasma. The matching circuit 16 conductsimpedance matching between the plasma electric discharge circuit in theprocessing chamber 2 and the high frequency electric power supplysection 17 in the case of generating this plasma.

[0044] The lower electrode 3 is connected to the electrostaticallyattracting DC electric power supply section 18 via RF filter 15. Whenthe electrostatically attracting DC electric power supply section 18 isdriven, as shown in FIG. 3(a), negative electric charges are accumulatedon the surface of the lower electrode 3. When plasma is generated in theprocessing chamber 2 by driving the high frequency electric power supplysection 17 as shown by the dotted portion 33 in FIG. 3(b), the DCcurrent impressing circuit 32 for connecting the semiconductor wafer 6,which is put on the holding face 3 g via the protective sheet 30, to thegrounding section 9 is formed in the processing chamber 2 via theplasma. Due to the foregoing, a closed circuit is formed in which thelower electrode 3, RF filter 15, the electrostatically attracting DCelectric power supply section 18, the grounding section 9, the plasmaand the semiconductor wafer 6 are successively connected in this order,and positive electric charges are accumulated on the semiconductor wafer6.

[0045] Coulomb's force acts between the negative electric charges, whichare accumulated on the holding face 3 g of the lower electrode 3 made ofconductive material, and the positive electric charges which areaccumulated on the semiconductor wafer 6 via the protective sheet 30containing the insulating layer which is a dielectric body. By thisCoulomb's force, the semiconductor wafer 6 is held by the lowerelectrode 3. At this time, RF filter 15 prevents the high frequencyvoltage of the high frequency electric power supply section 17 frombeing directly given to the electrostatically attracting DC electricpower supply section 18. In this connection, the polarity of theelectrostatically attracting DC electric power supply section 18 may bereversed.

[0046] In the above constitution, the electrostatically attracting DCelectric power supply section 18 composes the DC voltage impressingmeans for electrostatically attracting the semiconductor wafer 6 byutilizing Coulomb's force acting between the semiconductor wafer 6 andthe holding face 3 g of the lower electrode 3, which are separate fromeach other by the protective sheet 30, when DC voltage is impressed uponthe lower electrode 3. That is, concerning the holding means for holdingthe semiconductor wafer 6 on the lower electrode 3, the vacuumattracting means for attracting the protective sheet 30 via theplurality of attracting holes 3 e, which are open to the holding face 3g, by vacuum and the DC voltage impressing means described above areprovided, and these two types of means are properly used.

[0047] On the side of the processing chamber 2, there is provided anopening portion (not shown in the drawing), which can be freely openedand closed, for carrying in and out an object to be processed.

[0048] In the case of carrying in and out the semiconductor wafer 6, theupper electrode 4 is raised by the electrode elevating mechanism, and aconveyance space is ensured on the lower electrode 3. Under thiscondition, the semiconductor wafer 6 is carried in and out by the waferconveyance mechanism via the opening section.

[0049] The plasma processing device is composed as described above.Referring to FIG. 4 and the other drawings, explanations will be madeinto the method of manufacturing the semiconductor device, in which theabove plasma processing device is used, and the plasma processing methodcarried out in the process of the method of manufacturing thissemiconductor device.

[0050] First, in FIG. 4(a), reference numeral 6 is a semiconductorwafer, on which a plurality of semiconductor elements are formed, thethickness of which is reduced by machining. In the thickness reducingstep conducted before, the thickness is reduced to a value not more than100 μm. Before the thickness reducing step, the protective sheet 30 isattached onto the circuit forming face (the first face) of semiconductorwafer 6 (the sheet attaching step).

[0051] In this case, the profile of the protective sheet 30 is the sameas that of the semiconductor wafer 6 so that the protective sheet 30 cancover the overall circuit forming face 6 a and can not protrude outsidethe semiconductor wafer 6. Due to the foregoing, the protective sheet 30is not exposed to plasma in the plasma processing conducted later.Therefore, it is possible to prevent the protective sheet 30 from beingdamaged by plasma.

[0052] On the reverse face (the second face) of the circuit forming face6 a of the semiconductor wafer 6 after the thickness reducing step, theresist film 31 is formed covering the overall face of the semiconductorwafer 6. This resist film 31 is used for forming a mask to determine thecutting lines for dividing the semiconductor wafer 6 into individualpieces of the semiconductor elements. Patterning is conducted on theresist film 31 by means of photolithography so as to remove portions ofthe resist film 31 corresponding to the cutting lines 31 b. Due to theforegoing, on the reverse face of the semiconductor wafer 6, the mask isformed, the region except for the portions of the cutting lines 31 b ofwhich is covered with the resist film 31 a. The semiconductor wafer 6having the mask in this state becomes an object to be processed by meansof plasma processing (the mask formation step).

[0053] Referring to the flow chart shown in FIG. 5 and also referring toeach drawing, the plasma processing method, the object to be processedof which is this semiconductor wafer 6 having the mask, will beexplained below. First, the semiconductor wafer 6 having the mask isconveyed into the processing chamber 2 (ST1). Next, the vacuumattracting pump 12 is driven so as to attract from the attracting holes3 e by vacuum, and the vacuum attraction of the semiconductor wafer 6 isturned on and the electrostatically attracting DC electric power supplysection 18 is turned on (ST2). By this vacuum attraction, thesemiconductor wafer 6 is held by the lower electrode 3 while theprotective sheet 30 is being tightly contacted with the holding face 3 gof the lower electrode 3.

[0054] After that, the door of the processing chamber 2 is closed andthe upper electrode 4 is lowered (ST3). Due to the foregoing, theelectrode distance between the upper electrode 4 and the lower electrode3 is set at the electrode distance shown by the plasma processingcondition. Next, the vacuum pump 8 is set in motion so as to startdecompressing the processing chamber 2 (ST4). When the degree of vacuumin the processing chamber 2 has reached a predetermined value, theetching gas (the plasma dicing gas) composed of mixed gas containingsulfur hexafluoride and helium is supplied from the plasma generatinggas supply section 21 (ST5).

[0055] When the pressure in the processing chamber 2 has reached a valueshown in the plasma processing condition, the high frequency electricpower supply 18 is driven, and a high frequency voltage is impressedbetween the upper electrode 4 and the lower electrode 3, so that anelectric discharge of plasma is started (ST6). Due to the foregoing, theplasma dicing gas containing fluorine gas is transferred into a plasmastate in the electric discharge space between the upper electrode 4 andthe lower electrode 3. By the generation of plasma, fluorine gas such assulfur hexafluoride is irradiated from the mask side (the resist film 31a side) to the semiconductor wafer 6. By this irradiation of plasma,only portions of silicon of primary material of the semiconductor wafer6, which are portions of the cutting lines 31 b not covered with theresist film 31 a, are plasma-etched by plasma of fluorine gas.

[0056] At the same time, a DC electric current impression circuit isformed in the electric discharge space between the upper electrode 4 andthe lower electrode 3 as shown in FIG. 3. Due to the foregoing, anelectrostatically attracting force is generated between the lowerelectrode 3 and the semiconductor wafer 6, so that the semiconductorwafer 6 is held on the lower electrode 3 by the electrostaticallyattracting force. Therefore, the protective sheet 30 is tightlycontacted with the holding face 3 g of the lower electrode 3.Accordingly, the semiconductor wafer 6 can be stably held in the processof plasma processing. At the same time, the protective sheet 30 can beeffectively cooled by the cooling function provided by the lowerelectrode 3, so that the occurrence of heat damage generated by plasmaelectric discharge can be prevented.

[0057] When this plasma etching proceeds, as shown in FIG. 4(e), thecutting groove 6 d is formed only in a portion of the cutting line 31 bon the semiconductor wafer 6. When the depth of this cutting groove 6 dreaches the overall thickness of the semiconductor wafer 6, thesemiconductor wafer 6 is divided into individual pieces of thesemiconductor elements 6 c (the plasma dicing step).

[0058] Referring to FIG. 6, the progress of this plasma dicing will beexplained below. FIG. 6(a) is a view showing a state before the plasmadicing is started. The protective sheet 30 is attached onto the circuitforming face 6 a of the semiconductor wafer 6 via the adhesive layer 30c. When the plasma dicing is started and the plasma of fluorine gas isexposed from the mask side, portions of the cutting lines 31 b, whichare exposed to the plasma, are plasma-etched, and the cutting grooves 6d are formed inward the semiconductor wafer 6.

[0059]FIG. 6(b) is a view showing a state in which the plasma processingtime has passed after the plasma etching was started and the formationof the cutting grooves 6 d has proceeded. At this time, due to the wantof uniformity of the etching rate distribution in the processing chamber2, the progress of plasma etching fluctuates between the cutting grooves31 b.

[0060] For example, in FIG. 6(b), the cutting groove 6 d has alreadypenetrated the entire thickness of the semiconductor wafer 6 at theposition of the cutting line 31 b located on the right in FIG. 6(b),because the etching rate in this region is high. Therefore, plasmaetching proceeds to the adhesive layer 30 c exposed to the plasma. Onthe other hand, at the position of the cutting line 31 b located on theleft in the drawing, the cutting groove 6 d has not reached the lowerface of the semiconductor wafer 6 yet, because the etching rate in thisregion is low, that is, the cutting has not been completed in thisregion.

[0061] Concerning the formation of the cutting groove by plasma etching,the degree of progress of etching is not constant in the groove widthdirection. Therefore, a profile of the cross section of the lower endportion of the groove is formed into a V-shape in which the groovecenter protrudes downward. Therefore, as shown by the cutting groove 6 don the right in the drawing, even in the state in which the entirethickness of the semiconductor wafer 6 has been cut, the groove width ofthe cutting groove 6 d at the bottom portion is smaller than the widthof the cutting groove 31 b. In the dicing of the semiconductor wafer 6,it is preferable that the semiconductor wafer 6 is cut so that thecutting groove of the uniform groove width can be formed with respect tothe entire thickness of the semiconductor wafer 6. In order to form thecutting groove of the uniform groove width, according to theconventional method, it is necessary to make the cutting grooveexcessively proceed downward in the thickness direction. As a result,the protective sheet 30 is damaged by the heat generated in the processof plasma etching.

[0062]FIG. 6(c) is a view showing a state in which a longer plasmaprocessing time has passed. When plasma etching is conducted on the twocutting lines 31 b in the state shown in FIG. 6(b), first, on thecutting line 31 b located on the right in the drawing, the cuttinggroove 6 d proceeds to the entire thickness of the adhesive layer 30 cand reaches the metallic layer 30 b. Since the etching rate is very lowin the case where plasma of fluorine gas acts on the metallic layer 30c, the progress of the cutting groove 6 d in the thickness directionsubstantially stops on the surface of the metallic layer 30 b. On theother hand, plasma etching successively acts on the semiconductor wafer6 and the adhesive layer 30 c. Therefore, when plasma etching issuccessively conducted for a predetermined period of time, the groovewidth of the cutting groove at the bottom portion becomes the same asthe groove width of the cutting groove at the upper portion.Accordingly, the cutting groove 6 d of the uniform groove width can beformed.

[0063] In the process of successively conducting plasma etching asdescribed above, on the cutting line 31 b located on the left, which isin a region of a low etching rate, the cutting groove 6 d is formed withrespect to the entire thickness of the adhesive layer 30 c as the timepasses. In the same manner as described above, the progress of etchingin the thickness direction is stopped on the surface of the metalliclayer 30 b. When plasma etching is successively conducted after that, inthe same manner as described above, the groove width at the bottomportion of the cutting groove 6 d becomes the same as the groove widthat the upper portion. In this way, the cutting groove 6 d of the uniformgroove width can be formed.

[0064] As described above, the metallic layer 30 b provided on theprotective sheet 30 functions as an etching stop layer for stopping theprogress of groove formation in the thickness direction by plasmaetching. When this etching stop layer is provided, the occurrence ofheat damage caused on the protective sheet 30 by plasma can be preventedand the following effects can be provided.

[0065] Until plasma etching proceeds to a state shown in FIG. 6(c), thesemiconductor wafer 6 having the mask is successively exposed withplasma, and heat generated by the plasma concentrates on the bottomportion of the cutting groove 6 d at the position of the cutting line 31b. Therefore, in the state shown in FIG. 6(b), the protective sheet 30in the periphery of the bottom portion of the cutting groove 6 d isintensively heated. Even in the above state, since the metallic layer 30b, which functions as an etching stop layer, is provided on theprotective sheet 30 between the insulating layer 30 a and thesemiconductor wafer 6, plasma etching does not proceed downward to theinsulating layer 30 a.

[0066] Since the metallic layer 30 b is made of material of a high heatconductivity such as aluminum or copper, even if the periphery of thecutting groove 6 d is intensively heated, the heat diffuses to theentire face of the protective sheet. The thus diffused heat istransmitted to the holding face 3 g of the lower electrode 3 cooled by acooling means. Accordingly, the protective sheet is not superheated inthe process of plasma etching, that is, no problems are caused, andplasma dicing can be executed in a good condition.

[0067] When plasma processing is successively conducted for apredetermined period of time, plasma dicing is completed. Then, theelectric discharge of plasma is stopped (ST7). After that, the vacuumpump 8 is stopped (ST8), and the exhaust changeover valve 7 is changedover so that it can be opened to the atmospheric air (ST9). Due to theforegoing, pressure in the processing chamber returns to the atmosphericpressure. Then, the vacuum attraction is turned off and theelectrostatically attracting DC electric power supply is turned off(ST10). Due to the foregoing, the semiconductor wafer 6, which has beendivided into individual pieces of the semiconductor elements 6 c andheld on the protective tape 30, is released from the attraction.

[0068] After that, the semiconductor wafer 6, which has been subjectedto plasma processing, is carried out (ST11). While nitrogen gas is beingblown out from the attracting holes 3 e, the semiconductor wafer 6 isattracted and held by a handling mechanism not shown such as anattracting head and carried outside the processing chamber 2. In theprocess of plasma dicing, the protective sheet 30 is entirely coveredwith the semiconductor wafer 6 as described above. Therefore, no damagesuch as a heat deformation, which is caused when the protective sheet isexposed to plasma, is not generated. Accordingly, the protective sheet30 always comes into close contact with the holding face-0.3 g and thesemiconductor wafer 6, and it fulfills its function.

[0069] The semiconductor wafer 6 carried out together with theprotective sheet 30 is sent to the mask removing step. As shown in FIG.4(d), the resist film 31 a is removed from the individual pieces of thesemiconductor elements 6 c. Then, the semiconductor wafer 6 is sent tothe sheet peeling step, and the protective sheet 30 is peeled off fromthe circuit forming face of the semiconductor device obtained when thesemiconductor wafer 6 is divided onto individual pieces of thesemiconductor elements 6 c (the sheet peeling step). This sheet peelingstep is conducted in such a manner that the adhesive sheet 37 forholding is stuck onto the reverse faces of the semiconductor elements 6c so that each semiconductor element 6 c can be held on the adhesivesheet 37 as shown in FIG. 4(e).

[0070] As explained above, in the method of cutting a semiconductorwafer of the present embodiment, in the process of plasma dicing ofcutting the semiconductor wafer by plasma etching, a protective sheet onwhich a metallic layer, a plasma etching rate of which is lower thanthat of the semiconductor wafer, is provided on one face of aninsulating sheet which becomes the base material, and this metalliclayer is used as an etching stop layer for suppressing the progress ofetching.

[0071] Due to the foregoing, it is possible to solve the conventionalunsolved problem of heat damage which is caused on a protective sheetdue to the want of uniformity of the etching rate, that is, it ispossible to solve the problem of heat damage which is caused on aprotective sheet when the protective sheet on the lower face side of thesemiconductor wafer is directly exposed to plasma and silicon of thesemiconductor wafer is completely removed in a region of a high etchingrate.

[0072] This embodiment provides an example in which the plasma dicingstep is executed by utilizing one type mixed gas containing fluorinegas. However, the plasma dicing step may be executed while a pluralityof types of gasses are being changed over stepwise. For example, theconstitution of the plasma generating gas supply means and the processmay be changed in such a manner that the layer of SiO₂ of thesemiconductor wafer is etched by plasma of fluorine gas of hydrogenattaching and that the protective film (the passivation film) is etchedby plasma of oxygen gas.

[0073] According to the present invention, in a method of cutting asemiconductor wafer by means of plasma etching, a protective sheet onwhich a metallic layer, a plasma etching rate of which is lower thanthat of the semiconductor wafer, is formed is provided on one face of aninsulating sheet which becomes the base material, and this metalliclayer is used as an etching stop layer for suppressing the progress ofetching. Therefore, it is possible to realize plasma etching, theefficiency of which is high, the fluctuation of the progress of which isavoided. Further, it is possible to prevent a protective sheet frombeing damaged by heat generated in the process of cutting.

What is claimed is:
 1. A method of cutting a semiconductor wafer inwhich the semiconductor wafer, on the first face of which semiconductorelements are formed, is cut by plasma etching from the second faceopposite to the first face, comprising: a sheet attaching step ofattaching a protective sheet onto the first face; a mask formation stepof forming a mask to determine cutting lines on the second face forcutting the semiconductor wafer; and a plasma etching step of conductingplasma-etching on portions of the cutting lines by exposing thesemiconductor wafer to the plasma from the mask side, wherein theprotective sheet is composed of an insulating sheet, which becomes thebase material, and a metallic layer, a plasma etching rate of which islower than that of the semiconductor wafer, provided on one face of theinsulating sheet, and the metallic layer side is attached onto the firstface via an adhesive layer in the sheet attaching step.
 2. A method ofcutting a semiconductor wafer according to claim 1, wherein plasmagenerating gas used in the plasma dicing step contains at least gas offluorine, and the metallic layer contains either aluminum or copper. 3.A protective sheet used in a method of cutting a semiconductor wafer,the protective sheet being attached onto a first face of thesemiconductor wafer when the semiconductor wafer, on the first face ofwhich semiconductor elements are formed, is cut by plasma etching fromthe second face opposite to the first face, the protective sheetcomprising: an insulating sheet that becomes the base material; and ametallic layer provided on one face of the insulating sheet and made ofmetal, a plasma etching rate of which is lower than that of thesemiconductor wafer.
 4. A protective sheet used in a method of cutting asemiconductor wafer according to claim 3, wherein plasma generating gasused in the plasma etching contains at least gas of fluorine, and themetallic layer contains either aluminum or copper.
 5. A protective sheetused in a method of cutting a semiconductor wafer according to claim 3,wherein an adhesive layer is provided on a face of the metallic layer.6. A protective sheet used in a method of cutting a semiconductor waferaccording to claim 4, wherein an adhesive layer is provided on a face ofthe metallic layer.